Dampers are known which use a hydraulic fluid as the working medium to create damping forces to control motion, shock, and/or vibration. One special class of these devices is controllable. In particular, controllable dampers are known which include Electrorheological (ER) fluids, Electrophoretic (EP) fluids, Magnetorheological (MR) fluids, and Hydraulic fluids (Semi-active), etc. Examples of ER-type dampers may be found in U.S. Pat. No. 5,029,677 to Mitsui. Descriptions of EP-type dampers may be found in U.S. Pat. No. 5,018,606 to Carlson. Examples of Semi-Active hydraulic dampers and valves may be found in U.S. Pat. No. 3,807,678 to Karnopp et al. and U.S. Pat. No. 5,207,774 to Wolfe et al.
Of particular interest are Magnetorheological (MR) fluid dampers, as they only require small electrical currents (typically several amps or less) and do not present the potential shock hazard of ER devices, because they operate on much lower voltage (typically 12 volts or less). MR fluid dampers employ a controllable Magnetorheological (MR) fluid comprised of small softmagnetic particles dispersed within a liquid carrier. Typical particles include carbonyl iron, or the like, having various shapes, but which are preferably spherical, and which exhibit mean diameters of between about 0.1 .mu.m to 500 .mu.m, and more preferably between about 1 .mu.m and 100 .mu.m. The carrier fluids include various known hydraulic oils, and the like. These MR fluids exhibit a thickening behavior (a rheology change), sometimes referred to as an "apparent viscosity change", upon being exposed to a magnetic field of sufficient strength. The higher the magnetic field strength to which the MR fluid is exposed, the higher the damping force that can be achieved within the particular MR damper.
In particular, MR fluid devices provide ease of controllability through simple fluctuations in the electrical current supplied to the magnetic field generator (generally a wound-wire coil). Notably, MR fluids and dampers have demonstrated durability yet unobtained with ER devices (which exhibit a change in rheology upon being exposed to "electric" fields). Further, MR devices provide simplicity previously unachieved with controllable Semi-Active devices, in that the controllable valves have few, or no, moving parts. Descriptions of prior art MR dampers may be found in U.S. Ser. No. 08/674,179 entitled "Controllable Vibration Apparatus" and U.S. Pat. Nos. 5,492,312, 5,284,330, and 5,277,281, all of which are commonly assigned to the assignee of the present invention.
Recently, the use of MR dampers has been proposed for control of civil engineering structures for earthquake hazard mitigation. MR dampers offer the ability to be controlled using low power, thus, they are highly adaptable to battery power operation. Notably, loss of the main electrical power source during a seismic event is a reality that must be contended with, making MR dampers an attractive candidate. However, as in most applications, modest or low cost and high performance are required. Further, the fluids used must be able to withstand long stationary periods and not settle out. Therefore, there is a need for a low cost, reliable, and high performance damper for seismic applications.